Tagging of articles for identification and/or theft protection is known. For instance, many articles are identified using a bar code comprising coded information which is read by passing the bar code within view of a scanner. Many articles also include a resonant transponder or resonant tag for use in theft detection and prevention. More recently, passive resonant tags which return unique or semi-unique identification codes have been developed. These tags typically include an integrated circuit (IC) which stores the identification code. Such "intelligent" tags provide information about an article or person with which the tag is associated which is detected in the zone of an interrogator or reader. The tags are desirable because they can be interrogated rapidly, and from a distance. U.S. Pat. No. 5,446,447 (Carney et al.), U.S. Pat. No. 5,430,441 (Bickley et al.), and U.S. Pat. No. 5,347,263 (Carroll et al.) disclose three examples of such intelligent tags.
Radio frequency identification (RFID) tags or cards generally include a resonant antenna circuit electrically connected to the IC. The IC is essentially a programmable memory for storing digitally encoded information. The interrogator (transmit antenna) creates an electromagnetic field at the resonant frequency of the RFID tag. When the tag is placed into the field of the interrogator, an AC voltage is induced in the resonant antenna circuit of the tag, which is rectified by the IC to provide the IC with an internal DC voltage. As the tag moves into the field of the interrogator, the induced voltage increases. When the internal DC voltage reaches a level that assures proper operation of the IC, the IC outputs its stored data. To output its data, the IC creates a series of data pulses by switching in or out an extra capacitor or inductor across the antenna circuit for the duration of the pulse, which changes the resonant frequency of the tag, detuning the tag from the operational frequency. That is, the tag creates data pulses by detuning itself, which changes the amount of energy consumed by the tag. The interrogator detects the consumption of energy in its field and interprets the changes as data pulses.
Although such RFID tags or cards are known, there are still technical difficulties and limitations associated with the operation of such tags. One problem with attempting to read multiple RFID tags within an interrogation zone of the interrogator is that more than one tag may be activated by the interrogator at about the same time. When such tags are located proximate to each other, the fields generated by one tag can disturb the fields generated by another tag. This problem of mutual inductance is especially significant for RFID tags which transmit their information by detuning, as described above. As a consequence, the effective reading distance for the tags drops and the modulation of the tag can become completely ineffective due to the fact that such modulation depends upon the tag being in resonance (or close to it). Thus, such detuning caused by other tags can make the reading of stored information impossible or nearly impossible.
Long range reading applications require a highly modulated AM field. A high degree of AM modulation is obtained by providing the greatest field disturbance to the antenna's magnetic field. Maximum field disturbance (represented by a maximum amplitude difference) is obtained when the loading effect of the tag is completely removed after each field disturbance, and when the signal from a tag to be read is not mixed in with or attenuated by noise or interference from other tags. Thus, conventional schemes which do not effectively decouple tags from their environment cannot provide for long range reading.
One approach to minimizing the problem of an RFID tag generating fields which disturb or affect neighboring tags is described in copending U.S. application Ser. No. 09/035,027 filed Mar. 5, 1998, entitled "Apparatus for Magnetically Decoupling an RFID Tag," which is incorporated by reference in its entirety herein. In this approach, an RFID transponder includes an integrated circuit for storing data and an inductor electrically connected to the integrated circuit. The inductor includes a first coil electrically connected to a second coil. A resonant capacitor is electrically connected to the integrated circuit and to at least one of the first and second coils, such that the resonant capacitor and the at least one connected coil have a first predetermined resonant frequency. A switch having a first position and a second position is provided for selectively allowing current to flow through the second coil. When the switch is in the first position, exposure of the transponder to an external field at or near the first resonant frequency induces a voltage in the inductor and causes a first current to flow through the inductor in a first direction, thereby generating a local field. When the switch is in the second position, exposure of the transponder to an external field at or near the first resonant frequency induces a voltage in the inductor and causes a first current to flow through the first coil in a first direction, thereby generating a first local field and a second current to flow through the second coil in a second, opposite direction, thereby generating a second local field. A sum of the first and second local fields approaches zero.
This field canceling technique is feasible but has some disadvantages. For example, the circuit implementation uses a three turn coil in series with a one turn coil which requires approximately three times greater current flow in the one turn coil. This is difficult to achieve, especially when a low impedance switch must be connected across the one turn coil. The field canceling technique also limits the flexibility of the design because the mutual coupling between the coils is critical, and must be adjusted empirically.
Accordingly, there is a need for an alternative method of preventing RFID tags from generating fields which disturb or affect other nearby RFID tags or other resonant cards or tags. The present invention fulfills this need.